The cellular interactions between blastocyst and uterus during implantation will be studied. Preimplantation embryos will be obtained from nonhuman primates (rhesus monkey and baboon) by a uterine flushing method and examined by electron microscopy to determine the mechanism of blastocyst formation, zona loss, and syncytium formation. Postimplantation stages will be examined with special attention to the mechanisms of penetration of uterine epithelium, penetration of basal lamina and mechanisms of stromal progression of trophoblast. Ferritin-conjugated lectins and differential digestion will be used to examine the glycoc alyx of the blastocyst and uterine surface of the rat at implantation. Studies with simulated blastocysts will be used to further study the basis of implantation chamber formation and decidual inducation in the rat. The development of the decidua, characterization of the stromal matrix, and migration of trophoblast cells following the initiation of implantation will also be studied in the rat using cytochemical procedures.